CleanLogix Application Note AN1002
Scouring and Surface Preparation of Silicone
Rubbers (and other materials)
APPLICATION NOTE
By David Jackson
Application Note AN1002
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Introduction to Silicone Rubbers
Figure 1 shows a molecular model for polydimethylsiloxane or PDMS, a common
silicone polymer. Cured silicones are an elastomer (a rubber-like material)
composed of siloxanes; silicon together with carbon, hydrogen, and oxygen.
Silicone rubbers are widely used in industry, and there are multiple formulations.
Silicone rubbers are often one- or two-part polymers, and may contain fillers or
additives to improve properties or reduce cost. Silicone rubber is generally nonreactive, stable, and resistant to extreme environments and temperatures from
−55 °C to +300 °C while still maintaining its useful mechanical and chemical
properties. Due to these properties and its ease of manufacturing and shaping,
silicone rubbers can be found in a wide variety of products, including: aerospace
Figure 1
3-D Molecular Model for PDMS
applications such as electronic cabling, vibration dampers and electrical device
insulators; biomedical devices and implants; and pharmaceutical devices such as
seals, liners and septa. During manufacture, heat may be required to vulcanize
(set or cure) the silicone into its rubber-like form. This is normally carried out in a
two stage process at the point of manufacture into the desired shape, and then in
a prolonged post-cure process. Silicones can also be injection molded and can
include special monomer chemistries and properties such as fluorinated
chemistry, steam resistant, metal detectable, fluorescent, electrically conductive,
chemically resistant, low-smoke emitting, and flame-retardant. Example physical
properties of a silicone rubber (e.g., LSR Polymax 2005) are shown in Table 1.
Mechanical Properties
Hardness, shore A
Tensile strength
Elongation at break
Maximum temperature
Minimum temperature
10–90
11 N/mm²
100–1100%
+300 °C
−120 °C
Table 1
Example Silicone Rubber Properties
CleanLogix Application Note AN1002
Silicone Rubber Contamination
Following conventional liquid silicone rubber (LSR) molding and curing
processes, and depending upon the type and amount of additives, cure cycle
times and conditions; solidified rubber products can contain up to 10% (by
weight) residual unreacted monomer fragments (called “oligomers”), fillers and
other interstitial residues; more simply termed “silicone contamination” for the
purposes of this application note. Silicone contamination is generally non-toxic,
biocompatible, non-corrosive, and non-volatile under standard temperature and
pressure conditions (STP). However portions of it can migrate to the surface or
out of the material entirely, when the surface is cleaned or exposed to non-steady
state environmental conditions in accordance with Fick’s molecular diffusion
principle. Residual silicone contamination can usually be left within the bulk
solidified material for many industrial and commercial applications, and may in
fact be required for the desired performance properties. However silicone
contamination, in particular volatile or mobile forms, can be problematic in certain
applications or systems involving biomedical, aerospace, and pharmaceutical
devices employed in aqueous, solvent or vacuum environments. For example,
silicone oligomers (considered a “leachable” contaminant) can be extracted from
silicone rubbers located within aqueous or solvent environments and volatilized
(considered an “outgas” contaminant) when situated within hot and/or vacuum
environments.
Examples of situations and environments where silicone
contamination poses a potential threat include vacuum systems, thermal
systems, space environment, the human body, or as components of containers
used to store liquids. Moreover, silicone contamination can present problems
during manufacturing processes such as plasma surface modifications and
materials joining. Silicone contamination is also a contamination concern in
critical manufacturing environments, for example cleanrooms used to fabricate
semiconductor chips and hard disk drives.
Aerospace,
Pharmaceutical, and
Medical Products
Application Note AN1002
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For example, in spacecraft applications silicone oligomers will “outgas” from the
pores of the bulk material under the absolute vacuum of space, migrating along
internal thermal gradients created by the spacecraft rotation into and away from
the Sun.
This can relocate silicone contamination into or onto critical
optoelectronic systems such as navigational or observation devices, causing
potential optical surface obscuration. In another example, the performance of
heat exchange surfaces can be negatively affected if coated with thin heatinsulating layers of silicone contamination and other forms of “volatile
condensable matter” or VCM. In pharmaceutical applications, silicone
contamination can be leached from container seals or septa by solvent-based
diluents. In medical applications, silicone contamination can leach from a silicone
rubber device such as tubing or sleeves and cause unwanted cellular responses
or cellular adhesion problems. Still moreover, subsurface silicone contamination
will migrate from the interior of the bulk substrate and onto the surface regions
during vacuum-based processes such as plasma treatment, interfering with the
formation of clean functionalized surfaces or shortening the longevity of same.
Finally, surfaces containing silicone contamination interfere with adhesive
bonding mechanisms, preventing the formation of strong chemical and
mechanical adhesive bonds.
One of the difficulties in modifying silicone surfaces relates to the mobile nature of
amorphous polymer molecules. For example, during surface modification, for
example oxidation, of a silicone rubber surface, molecular motions can (over a
period of time) cause the modified surface to intermingle and diffuse into the
CleanLogix Application Note AN1002
polymer matrix. This tendency is most pronounced in silicone elastomers which
have very mobile polymer chains. To overcome this problem, plasma treatments
may be used to crosslink and stabilize the polymeric surface. However, within
hours or even minutes after plasma treatment, the surface begins to revert back
to its original hydrophobic state. Uncrosslinked oligomers and low molecular
weight oils begin to migrate to the polymeric surface in accordance with Fick’s law
of molecular diffusion - migration of polymers from interior regions of high
concentration to exterior surface regions having a low concentration. These oils
tend to interfere with attachment or grafting of coatings to the polymer surface in
the case of plasma coating. These oils also prevent the formation of strong
chemical and mechanical adhesive bonds in adhesive bonding applications.
Conventional Silicone Rubber Treatment Solutions
There are generally three conventional treatment methods used to clean, scour,
or fully-react silicone substrates, respectively, described as follows:
•
•
•
Solvent Extraction
Thermal Vacuum Bakeout
Enhanced Silicone Curing Processes
Solvent extraction using organic solvents or solvent blends can be a very slow
process, and can alter the mechanical properties of certain silicone devices.
Extraction with toxic or flammable solvents on a large scale is dangerous and
consumes significant amounts of energy. Moreover, residual extraction solvent
residues left in the pores of the silicone rubber can be problematic to the function
or performance of the extracted device.
An alternative to solvent extraction is thermal vacuum bakeout or TVB. The TVB
process extracts oligomers from the bulk polymer using a high vacuum (i.e. less
-5
than 1x10 Torr) and heat (i.e., 125 deg. C) over an extended period of time.
TVB extraction proceeds somewhat slowly - for example up to 80 hours or more and usually degrades the mechanical properties of the silicone rubber due to
accelerated thermal aging. In addition, TVB-treated silicone rubbers can burn or
exhibit color change.
Conventional Solvent
Extraction using a
“Soxhlet Extractor”
An alternative to post-treat methods is more complete vulcanizing or curing to
convert most of the silicone monomer into solid polymer. These methods include
peroxide (free radical) and platinum (addition) curing techniques. However, these
curing processes create constraints as well. Peroxide curing can lead to
microscopic bubbles, surface darkening and a tackier surface (i.e., can pick up
more dirt). Platinum curing is a cleaner but a more expensive treatment process
and is typically used only in critical applications such as medical implants – socalled medical grade silicones. Also, platinum-cured silicones exhibit higher wear
rates and spallation relative to peroxide-cured silicones when used in mechanical
devices such as pumps.
As such, all conventional silicone rubber treatment methods (i.e., post-treatment
and curing methods) offer tradeoffs in terms of processing time, life-cycle costs
and end-product performance.
Application Note AN1002
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A newer treatment technique that is generally unknown or not well understood by
silicone device manufacturers is carbon dioxide processing, or simply “CO2
Processing”. CO2 processing uniquely employs one or a combination of solid,
liquid, supercritical, and plasma CO2 chemistries and processes to treat silicones,
CleanLogix Application Note AN1002
as well as many other types of substrates requiring dry scouring and/or surface
treatment. CO2 processing offers silicone device manufacturers a robust, lean
and green cleaning (and surface treatment) option that can provide distinct
economic and performance advantages relative to the conventional treatment
processes. CO2 processing of silicone rubbers using supercritical and liquid CO2
was first developed in the 1980’s at Hughes Aircraft to prepare materials for use
in high reliability commercial and military communications satellites and spacebased warfare systems, enabling silicone devices to operate in the harsh
environment of space without contaminating and degrading the performance of
critical flight hardware.
The CO2 Alternative
Our CO2 processing technology provides silicone (and other biomedical polymer)
device manufacturers a robust platform of post-cure or post-process substrate
treatment options, and includes:
•
•
•
Centrifugal CO2™ Immersion-Extraction Cleaning
CO2 Composite Spray™ Cleaning
CO2 Plasma Blast™ Surface Modification
Centrifugal CO2™ Immersion-Extraction Cleaning
Dense phase (liquid and supercritical) CO2 is a non-toxic solvent that dissolves
many types of organic films and oils, and is particularly suited to dissolving and
removing unreacted silicone oils. Near-zero or zero (supercritical state) surface
tension and low viscosity allows dense phase CO2 to penetrate microscopic pores
and crevices, delivering solvent cleaning power deep into the interior of silicone
rubber. With our patented Centrifugal CO2™ immersion-extraction cleaning
processes, contamination is rapidly removed from silicone rubber under both
physical (centrifugal pumping and scouring) and chemical cleaning actions.
Figure 2 shows a typical immersion-extraction profile for PDMS using a
Centrifugal CO2 cleaning process employing liquid CO2.to meet ASTM E 595
outgas performance criteria - total mass loss (TML) of < 1.00 % and collected
volatile condensable material (CVCM) of < 0.10%.
CO2 Treatment
Chemistries
Solid, Liquid,
Supercritical and
Plasma
(Top-to-Bottom)
Application Note AN1002
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Centrifugal CO2 cleaning processes employ either liquid or supercritical CO2 and
require no special permits even from the tough SCAQMD or EPA because it is
non-toxic, non-flammable, and non-VOC. Integrated CO2 distillation recycles
nearly 100% of the CO2 extraction solvent while separating and concentrating
silicone oils (and solvent modifiers if present). Optionally, Centrifugal CO2
processes can be hybridized with other treatment processes such as lowpressure plasma to provide cleaned-surface modifications, bio-burden reduction,
or to assist with outgassing and decomposing residual interstitial contaminants.
Any “Industry Approved” or “Mil-Spec” extraction cleaning solvent additive may be
used with the Centrifugal CO2 process as a “carbonated” prewash-extraction
agent or CO2 chemistry modifier, providing numerous novel and dry immersionextraction cleaning chemistries. This would be important when a particular
cleaning-extraction chemistry is needed that mimics a particular solvent
environment (polar, non-polar, ionic) in which the processed substrate will be
exposed; for example a specific pharmaceutical drug solvent carrier. This allows
for the continued use of a “spec’d in” immersion-extraction cleaning solvent
chemistry – but in a safer and more robust cleaning process.
CleanLogix Application Note AN1002
Finally, our CO2 Composite Spray™ and CO2 Plasma Blast™ Particle-Plasma
surface treatment processes serve as adjuncts or alternatives to the Centrifugal
CO2 process; providing a very robust surface treatment platform capability, and
are described below.
Silicone Extraction Profile
2.50%
% TML
2.00%
1.50%
ASTM E 595
1.00%
TML
CVCM
WVR
0.50%
0.77%
0.01%
0.29%
0.00%
0
10
20
30
40
50
Time (minutes)
Figure 2
TML Profile for PDMS
CO2 Composite Spray™ Cleaning
CO2 Composite
Spray™
Treatment
The cleaning phenomenon involved in our CO2 Composite Spray™ cleaning
technology are analogous to (line-of-sight) high-pressure spray cleaning using
halogenated solvents such as HFE 7100. For example, one similarity is the high
density (1.6 g/cm3) of the solid CO2 solid particles entrained in an inert heated
propellant fluid such as CDA. CO2 Composite Spray surface cleaning is a
nonabrasive process because of the low hardness of the CO2 particles (less than
1 Mohs) as compared to most conventional manufacturing substrates. Moreover,
and similar to high-pressure solvent spray cleaning, CO2 Composite Spray
cleaning provides physical momentum transfer (shear stress) and unique phase
change phenomenon (solidliquid) – providing scouring liquid solvent cleaning
action at the contact surface to simultaneously remove both particulate and thin
film oily contamination from a surface.
A CO2 Composite Spray exhibits halogenated solvent-like chemistry which
provides both physical scouring and chemical cleaning action for contaminated
surfaces. A CO2 Composite Spray can be adjusted in several dimensions – with
key process variables including pressure, temperature, particle size and
concentration – to provide a range of impact shear stresses and cleaning effects.
In addition, a CO2 Composite Spray can be modified with liquid, gas and solid
additives to provide a range of surface cleaning chemistries and modification
capabilities. Figure 3 demonstrates the removal of microscopic inorganic metal
oxidation residues from the surfaces of a laser welded titanium neurostimulator
lead using a CO2 Composite Spray.
Application Note AN1002
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Finally, the CO2 Composite Spray process can be further enhanced by
hybridizing it with atmospheric plasma - a patented and patents-pending method
called CO2 Plasma Blast™.
CleanLogix Application Note AN1002
Figure 3
Removal of Ti Weld Soot (Metal Oxides on Neurostimulator Lead)
CO2 Plasma Blast™ Treatment
The surface scouring and solvent cleaning actions of a CO2 Composite Spray™
are used in cooperation with atmospheric plasma CO2 (and other types of
plasma) to form a very robust surface cleaning and preparation treatment called
The CO2 Plasma Blast™ process is an atmospheric
CO2 Plasma Blast™.
hybrid particle-plasma surface ablation (i.e., treatment, transformation and
modification) process that combines electron and/or photon driven surface
ablation phenomena comprising an ionizing-heating plasma plume with
simultaneous surface scouring and cooling actions provided by the CO2
Composite Spray. Figure 4 demonstrates the significant improvement in light
cured acrylic adhesive bond strength for a low surface energy polymer (e.g.,
LDPE) using the CO2 Plasma Blast treatment process.
CO2 Particle-Plasma
Treatment
In this patent-pending surface treatment technique, the CO2 Composite Spray is
used to both precisely control surface temperature and cleanliness; the
simultaneous removal of heat contamination and processing debris such as
oxidation residues, gases, and ablated surface particles generated by the
atmospheric plasma process. Working in cooperation, atmospheric plasma
cracks and chemically alters the surface while CO2 particles and fluids
simultaneously vector surface debris and excess heat from the treated surface.
Application Note AN1002
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Figure 4
Improvement in Shear Strength
CleanLogix Application Note AN1002
Conclusion
Conventional pre- and post-treatment options for silicone rubbers pose different
constraints in terms cost of ownership, environmental, and material performance.
The CO2 alternative offers a robust platform for processing silicone rubbers for
medical, aerospace and pharmaceutical applications, as well as many other
types of polymers and surfaces requiring scouring and surface modification for
manufacturing processes such as bonding, coating and precision assembly. For
example, low energy polymers such as LDPE have been processed successfully
using CO2 processes.
Applicable Industries
Aerospace/Defense
Medical
Electro-Optical
Microelectronic
Applicable CO2 Technology
Centrifugal CO2™
CO2 Plasma Technology
CO2 Composite Spray™ Technology
Related Assembly Products/Processes
Surface Cleaning
Thermal Vacuum Bakeout
Solvent Extraction
Adhesive Bonding, Surface Modification and Coating
Select Industry Testing Standards
ASTM E 595
References
CO2 Processing
Application Note AN1002
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1. “Effect of Sterilization on the Mechanical Properties of Silicone Rubbers”,
Saint-Gobain Performance Plastics, Northboro R&D Center
2. ASTM E 595 Outgas Test Reports (Silicone Cable Extraction), Pacific Testing
Laboratories, Valencia, CA, Customer/Application Confidential
3. ASTM E 595-93, Standard Test Method for Total Mass Loss and Collected
Volatile Condensable Materials from Outgassing in a Vacuum Environment
4. “Peroxide or Platinum? Cure System Considerations for Silicone Tubing
Applications”, Dow Corning Healthcare
5. “Anatomy of an Ethylene oxide Sterilization Process”, Technical Tip #10,
Steris Isomedix Services
6. “The Leachable Challenge in Polymers used for Pharmaceutical Applications”,
Rubber World, Nov 2008.
Keywords
Silicone Rubber, TVB, Outgas, TML, CVCM, WVR, ASTM E 595
CleanLogix Application Note AN1002
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Centrifugal CO2, CO2 Composite Spray, and
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